In a prior-art fixed resistor network of this type having, for example, four film resistors such as that shown in FIGS. 12 to 14, an insulating substrate 1′ with a rectangular shape of length L and width W as seen in plan view has four film resistors 2′ arranged on a top surface of the substrate 1′ in the lengthwise direction thereof and has, for both ends of each film resistor 2′, terminal electrodes 4′ which are formed on both lengthwise sidewalls 3′ of the insulating substrate 1′. The fixed resistor network is surface mounted on a printed board by soldering each of the terminal electrodes 4′.
In addition, the prior art provides recesses 5′ in areas between the terminal electrodes 4′ on both lengthwise sidewalls 3′ of the insulating substrate 1′. When individual terminal electrodes 4′ are formed on both lengthwise sidewalls 3′, the individual recesses 5′ reliably separate these individual terminal electrodes 4′ from each other (see, for example, Japanese Examined Patent Publication No. S6-18123). A cover coat 7′ made of a material such as glass is formed on the top surface of the insulating substrate 1′ so as to cover each of the film resistors 2′.
Fixed resistor networks having the above construction must satisfy the following conditions.
(a) When the fixed resistor network is mounted on a printed board by soldering, of the individual terminal electrodes 4′ for both ends of the individual film resistors 2′, the occurrence of solder bridging between neighboring terminal electrodes 4′ must be low.
(b) When individual terminal electrodes 4′ are formed on both lengthwise sidewalls 3′ of the insulating substrate 1′ by the application and drying or firing of a conductive paste, connection of the conductive paste within a recess 5′ between neighboring terminal electrodes 4′ must be infrequent.
(c) The occurrence of the chipping or breaking off of material in terminal-electrode-forming areas 6′ between recesses 5′ on the lengthwise sidewalls 3′ of insulating substrate 1′ must be low.
(d) The frequency of cracking of the insulating substrate 1′ at the recesses 5′ must be low.
To address these requirements, prior-art fixed resistor networks have a construction in which, letting the pitch of the terminal electrodes 4′ along the lengthwise sidewalls 3′ of the insulating substrate 1′ be P, the width A′ at areas 6′ where the terminal electrodes 4′ are formed (referred to below as “terminal-electrode-forming areas”) between the recesses 5′ on the lengthwise sidewalls 3′ has been set to A′=approx. 0.6×P; the width B′ along the lengthwise direction 3′ of insulating substrate 1′ at the recesses 5′has been set to B′=approx. 0.4×P, and the depth C′ at each recess 5′ from the lengthwise direction 3′ has been set approximately equal to the width A′ at the terminal-electrode-forming areas 6′.
This type of construction has led to the following sorts of problems.
That is, in fixed resistor networks of a size where the pitch P between the terminal electrodes 4′ has been set to 0.5 mm, the width B′ of the recesses 5′ becomes 0.4×0.5=approx. 0.2 mm, which is small. When these fixed network chip resistors are surface mounted by soldering onto a printed board or the like, there is a strong possibility that solder bridging will occur so as to connect, of the above individual terminal electrodes 4′, neighboring terminal electrodes 4′ separated by a recess 5′.
In fixed resistor networks of a size where the pitch P between the terminal electrodes 4′ has been set to 0.4 mm, the width B′ of the recesses 5′ becomes B′=0.4×0.4=approx. 0.16 mm, which is even smaller. During solder mounting, the occurrence of solder bridges between neighboring terminal electrodes 4′ in such cases becomes very high.
Moreover, in fixed resistor networks of a size where the pitch P has been set to 0.4 mm, the depth C′ at each recess 5′ is C′=0.24 mm. At a resistor size having a pitch P of 0.4 mm, the width W of the insulating substrate 1′ is 0.6 mm, and so the depth C′ to width W ratio for the insulating substrate 1′ becomes large, resulting in frequent cracking of the insulating substrate 1′ at these recesses 5′. Furthermore, as the depth C′ at the recesses 5′ increases, the projecting length at the terminal-electrode-forming areas 6′ positioned between these recesses 5′ increases, resulting in more frequent chipping or breaking off of material in these terminal-electrode-forming areas 6′.
On the other hand, as the depth C′ at the recesses 5′ decreases, when the terminal electrodes 4′ are formed by applying a conductive paste to the terminal-electrode-forming areas 6′ situated between the recesses 5′, the conductive paste often extends into the recess 5′ from both sides and connects therein.
It is an object of the invention to provide a fixed resistor network which overcomes these problems.